Right now, Mozilla Firefox extensions require XUL support to run. This makes it very difficult to implement in non-Gecko browsers.

However, Mozilla is currently developing an extension API that uses just HTML, CSS, and JavaScript. While their main goal is to be compatible across different major versions of Firefox, a side benefit would be that it can be implemented by other browsers as well.

While the final JetPack specification will be released with Firefox 4.0 (currently targeted to be released in Q4 2010) prereleases are available right now that should at least give an overview which preconditions are required to be compatible with those extensions and whether it's even feasible to implement that API.

Similar to JetPack, Chrome's extension system in built on HTML, CSS, and JavaScript. The advantage compared to JetPack is, that it's already available and in use. However, Chrome enjoys only a fraction of Firefox' market share which could mean that in the future JetPack will be more popular than Chrome's API.

If not too complicated, maybe even compatibility with JetPack and Chrome is possible.